1. Field of the Invention
The present invention relates to a hot plate for use with a molten metal storage container that stores a molten metal temporarily and which keeps the molten metal temporarily stored in the storage container in its molten state by heating it to a particular constant temperature adequate for a subsequent casting operation, thus preventing the temperature of the molten metal from falling below the specific temperature prior to the casting operation.
2. Description of the Prior Art
In the casting field, metals having a relatively low fusing point, such as aluminum, zinc, copper, and an alloy of any ones of those metals, are temporarily stored in a molten state in a storage container prior to any subsequent casting operation, and any suitable heater is provided in the storage container for keeping those molten metals (which may be referred to as "molten bath", which term will be used in some instances herein and should be understood to have the same meaning as the molten metal) at the particular constant temperature adequate for the subsequent casting operation. The heater is available in various types, such as a radiant heater system, an electric heater system, and the like. There is another heater system that includes a hot plate which is immersed in the molten bath. This hot plate heater system is known to be better than any other prior art heater system, in that it provides the best thermal efficiency, and has no adverse effect on the quality of the molten metal while it is temporarily stored in the storage container.
An example of the heater system of the type including the hot plate that may be immersed in the molten bath is disclosed in a Japanese utility model application now published under No. 2 (1990)-1556. The hot plate includes an electric heating unit which is directly packaged in a ceramics material formed to an appropriate shape. The hot plate may be mounted on the inner wall of a furnace located below the molten bath or may be directly immersed in the molten bath. In the hot plate according to the above application, however, the electric heating unit that includes an electric heating wire or coil, such as nichrome wires, is embedded in electrocasted magnesia shaped as a flat plate compactly packaged in the ceramics material. The hot plate includes materials that have different coefficients of thermal expansion, which may cause great thermal deformation when the hot plate is heated by making the heating coil conduct current, which in turn may cause cracks or breakages in the hot plate. It is possible that any cracks would damage the electrical insulation, leading to a disastrous accident.